Current systems include flat plate solar panels, common in solar water heating, evacuated tube solar collectors, also used for hot water and other uses, and parabolic trough collectors. Less common are circular parabolic collectors. The parabolic collectors are examples of concentrating designs as are the large scale systems where a large number of flat mirrors reflect solar energy to a tower for purposes of generating steam for power generation. Each of these systems has limitations.
Flat panel solar collectors, such as those used in domestic hot water heating systems, make use of greenhouse effect to heat up an enclosed box with a transparent cover exposed to the sun. Both direct and indirect energy are collected. Limitations of these flat panel collectors include the fact that these panels are typically stationary and at hours of the day where the sun is not directly overhead are receiving only a fraction of the energy they could receive because of the oblique angle of the sun with respect to the panel normal. That is, the flat panels do not follow the sun to get the full exposure possible. Generally this represents a loss of up to 20-30% on a typical day. Overall, flat panel efficiencies typically vary from 40-60%. It is possible for a flat panel system to be designed to follow the sun, but the cost would be relatively high. Typically, flat panel collectors heat up the working fluid to around 70-80° C.
Parabolic trough collectors concentrate solar energy to a receiver tube oriented at the focal point of the parabola. Typically, these systems have been used for large energy producing facilities and use rather high concentration ratios in order to raise the temperatures to several hundred degrees. At such high temperatures, there are losses due to conduction, convection, and radiation. Surrounding the receiver with a glass tube virtually eliminates convection losses leaving losses only due to conduction and radiation, with the latter being larger at the higher range of temperatures. Since the losses are proportional to the exposed area, the receiver tubes are made small. As a consequence, the tracking accuracy must be quite high and, since the structures are large for the power required, such systems must also have a structure that can withstand distortions due to wind loading. Both the tracking and the structure contribute greatly to a higher cost. Apart from these, another factor reducing the overall efficiency of prior art parabolic trough collectors is the inability to absorb indirect radiation unlike the flat panel and evacuated tube collectors.
Efficient use of solar energy has been a goal for as long as using solar energy has been around. The temperature of fluid leaving the receiver is the usual variable and is adjusted using flow rate of the fluid. But the level of heating is dependent on the efficiency of the heat transfer. By increasing the flow rate on a given geometry, the efficiency also changes. One can flow so fast that it does not absorb enough energy. Or it can flow so slowly that the fluid begins to radiate away much of the previously absorbed heat. The efficiency versus speed is thus a peaked function with excess losses at each end.
Prior art parabolic trough concentrating solar systems attempt to minimize radiation energy loss by making the diameter of the receiver as small as possible. However, this causes several problems, including increasing the requirement of the coupled solar tracking sub-system to avoid thermal cycling losses.
It is the intention of this disclosure to present a highly efficient solar collector that is scalable from residential use to larger commercial enterprises, and can be used with the same high efficiency for virtually any of several purposes for which solar energy is to provide power. These purposes are primarily residential or commercial hot water heating including manufacturing processes that use hot water, low pressure steam, power generation, air conditioning, and space heating.
It is a further intention of this disclosure to provide a system which is inexpensive in construction, in the choice of materials, labor cost, shipping cost, maintenance cost and lifetime costs. No currently existing solar collector can provide the consistently high efficiency of the disclosed system across such a spectrum and maintain a low cost. But by combining several existing concepts, however, such a system can be devised.
Efficiency is defined in several ways, but for purposes of this disclosure, solar power efficiency is defined as the fraction of solar power utilized from the solar power directly available, per unit area. It is also desirable for the solar collector to occupy the smallest space possible so that roof tops of apartment buildings, for example, may be able to sustain a large number of collectors for all the apartments building.
Not included in the efficiency calculation is the extra solar radiation available indirectly. This energy comes from radiation scattered by the atmosphere and that reflected off of nearby surfaces such as buildings, walls, bodies of water, and so forth. This energy normally amounts to another 20-30% of the energy available directly from the sun. It is a goal of the present system to be able to absorb some of this energy as well.
What is needed is a modular solar system, facilitated to collect a substantial portion of the direct and indirect solar energy incident on the area occupied by the collector and use it to either generate electricity, or heat working fluid, or both. It would be advantageous to have a medium size or large modular solar system that fit on standard containers and can thereby be transported to the operation field location in separate modules, and that can be assembled on site within a few hours.